Module types 

The signature type system: a transform is a typed function over Arrow data. Column types are base types today; value-types layer on as richer signatures later. FvType is the language-neutral name in a transform.toml; to_arrow() is the binding to the engine’s Arrow line, so the contract stays authorable without an Arrow dependency in the manifest itself.

Structs

ColumnSpec

One column in a signature: a name + its base type.

SchemaSpec

A named tabular signature (one input dataset, or the single output). name is optional for the output; required-and-distinct for multi-input transforms (validated in the manifest).

Enums

FvType

Base column types the contract admits. Kept deliberately small; extended with value-types (currency, geo, classification) on the same seam later.